The term "kaolin" as it is used herein relates to near-white clay deposits which are dominantly comprised of the mineral kaolinite having the formula Al.sub.4 Si.sub.4 O.sub.10 (OH).sub.8, possibly with lesser amounts of the kaolin-group mineral, halloysite. Geologically, kaolins appear in the form of deposits of relatively fine particle size, resulting from the weathering of feldspathic rocks. Kaolin deposits can be classified as primary and secondary. Primary kaolins originate from the weathering of rocks such as granite or granite gneiss which contain high concentrations of feldspathic minerals and are found in the location where they were formed. Secondary kaolins originate from being borne along by river or lake freshwater currents, with subsequent settling and deposition in association with with other minerals such as quartz and mica.
Kaolin is used widely as a pigment, filler, coater, extender, ceramic raw material, catalyst base, electrical insulator, and pharmaceutical. Some more prominent uses include paper filling and coating; paint, plastic, adhesive, and ink pigment; rubber reinforcing agent; ceramic raw material from porcelain, dinnerware, tile, and enamels; catalyst base for petroleum cracking and auto exhaust emission catalytic control devices; cosmetics base; and digestive coating remedy.
Crude kaolin, however, typically contains various impurities which cause discoloration, most which are removed in order to make a commercially useful product, particularly as a paper coating. The coarser impurities, generally quartz, muscovite, and heavy minerals, are separated by settling or screening. However, there are usually micron-size impurities such as anatase, limonite, hematite, pyrite, organic matter, illite (hydromuscovite), montmorillonite (smectite), muscovite, and chlorite/biotite micas, etc. The partial or complete removal of these impurities in a economical manner has been the subject of much research and process equipment development in the industry, particularly with respect to improving whiteness and brightness of kaolin clay.
For many years, the only available method to maintain brightness and whiteness was that of selection. Only those clays which are naturally white and bright were mined, and the vast bulk of clay were simply by-passed or discarded. A technique suitable to produce a product for use in the paper industry has been to deflocculate the parent ore in an aqueous suspension and to separate the fine and coarse particles by a sedimentation process; either gravity or centrifugal. After separation the fine and coarse fractions were treated by various processes to enhance their usefulness to the paper industry. Another technique is chemical leaching using a clay reducing agent such as zinc or sodium hydrosulfite.
Only a very small percentage of the available clay, however, could be processed in any of these manners. Recognizing this, the industry has sought means for recovering, beneficiating, and using a greater percentage of natural kaolin clay. As a result, there have been, in recent years, many techniques developed for beneficiating and improving the whiteness and brightness of kaolin clay products. These techniques involve treatment of both the parent ore or a fraction of the parent ore and have been primarily developed for the beneficiation of kaolin clay products useful to the paper industry. For example, U.S. Pat. No. 3,471,011 to Iannicelli et al. proposes magnetic beneficiation utilizing techniques wherein a slurry of kaolin is subjected to a high intensity magnetic field of at least 8500 gauss. U.S. Pat. No. 3,414,422 to Iannicelli et al. proposes treating a kaolin slurry with a concentrated mineral acid, followed by subjecting the kaolin/acid mixture to high shear at an elevated temperature and washing the acid from the kaolin.
U.S. Pat. No. 5,128,027 to Halaka et al. proposes a process for removing mineral slimes from a slurry of kaolin comprising treating the kaolin slurry with a dispersing agent (e.g., polyphosphate) in the amount in excess of the amount required to obtain a minimum slurry viscosity, followed by recovering the kaolin fraction of the slurry by sedimentation from the supernatant slurry containing the fine particle fraction. U.S. Pat. No. 5,190,900 to Toro et al. relates to a chemical process for removing ferric iron present in small quantities in concentrates of various materials including kaolin. The process suspends the kaolin or other material in a bath comprising a 0.1 to 5M solution of an acid (e.g., hydrochloric acid) and a sugar such as fructose. The process of the reference enables the iron present as ferric iron (Fe.sup.3+) in the material to be removed by washing. U.S. Pat. No. 5,242,874 to De Oliveira et al. proposes a process for whitening kaolin in which aluminum powder, which is previously dissolved in hydrochloric acid, is added to remove a film of Al.sub.2 O.sub.3 which helps in the removal of iron.
These techniques, however, have limitations particularly in that the impurities are always removed and there is a weight loss of kaolin during processing. Moreover, these techniques do not work for the specific impurity, chlorite/biotite micas, that are sometimes found in kaolin deposits, and are of particular interest herein. Chlorites and biotites are widely distributed through Minnesota kaolin and tint the kaolin greenish-gray. Thus, there remains a need for a technique to brighten and whiten this type of kaolin.